Downhole removal of h2s and co2

ABSTRACT

A process for the removal of acid gas from fluid produced from a subterranean formation into a subterranean well includes the steps of producing fluid from the subterranean formation into an area selected from the group consisting of the subterranean well, a production tube in the well, an annular space defined therebetween, and combinations thereof so as to produce a production fluid containing acid gas; and introducing an aqueous solution of an active ingredient selected from the group consisting of Na 2 CO 3 , NaOH, and combinations thereof into the area so as to contact the aqueous solution with the fluid as the fluid travels from a down-hole production tube to a surface well facility so as to contact the active ingredient with the acid gas and thereby remove the acid gas from the production fluid.

BACKGROUND OF THE INVENTION

Environmental concerns have driven the need to reduce acid gas containing compounds from subterranean fluids because they are known to produce acid rain and airborne particulate material. State and local governments have enacted regulations including timetables for the development of one or more treatment methods which are capable of abating the problem. Further, the presence of acid gases such as CO₂ and H₂S is detrimental in the use of various commercial fluids. During processing, fluids containing acid gas are also responsible for accelerated corrosion times in tooling and instrumentation. The need for a really effective method of removing acid gases from geothermal fluids is clear.

Na₂CO₃ and other sodium salts are known to be used in aqueous solutions to scrub H₂S and CO₂. Methods exist today where additives are injected in multiple stages in facilities that are above ground in order to purge the fluids of the acidic gases. Although these methods have been improving over time, to ensure the removal of the gases, the fluids still have to be transported to other facilities. This subjects the machinery and transportation facility and equipment to corrosive gases while endangering the people who operate them.

SUMMARY OF THE INVENTION

In a hydrocarbon production environment, one possible solution would be to find a way to perform the scrubbing operation downhole, inside the well. However the issue that prevents such use is the possibility of damage to the formation. The contact of the Na₂CO₃ with the formation can lead to changes of the properties of the porous media, resulting in damage to the formation and reduction in well production. The primary object of the invention is to further address the need for effective acid gas scrubbing.

According to the invention, the foregoing objects and advantages have been attained.

According to the invention, a process for the removal of acid gas from fluid produced from a subterranean formation into a subterranean well is provided which comprises the steps of: producing fluid from the subterranean formation into an area selected from the group consisting of the subterranean well, a production tube in the well, an annular space defined therebetween, and combinations thereof so as to produce a production fluid containing acid gas; and introducing an aqueous solution of an active ingredient selected from the group consisting of Na₂CO₃, NaOH, and combinations thereof into said area, so as to contact the aqueous solution with the fluid as the fluid travels from a down-hole production tube to a surface well facility so as to contact the material with the acid gas and thereby remove the acid gas from the production fluid.

By controlling the amount of solution and material injected, and maintaining a proper pressure balance in the well, contact between the solution and the formation can be prevented.

BRIEF DESCRIPTION OF THE DRAWINGS

A detailed description of preferred embodiments of the invention follows with reference to the attached drawing, wherein:

FIG. 1 illustrates an application of a method according to the invention inside the well.

DETAILED DESCRIPTION

The invention relates to improvements in methods of scrubbing hydrocarbon fluids for H₂S, CO₂ and other acid gases.

As set forth above, surface scrubbing of fluids to remove acid gases can be effective at removing the acid gases, but exposes facilities for transportation of the fluids to the location where acid gas is removed to damage and corrosion from the acid gases. In accordance with the present invention, in the environment of downhole production of hydrocarbon fluids, many such fluids contain acid gases as discussed. In accordance with the present invention, the fluid with acid gas can be treated downhole with a scrubbing additive, and careful monitoring of the amount of additive to be used and of the pressure in the well as compared to that of the formation can result in effective downhole treatment without exposing the formation to damage from the scrubbing solution.

According to the invention, aqueous solutions of Na₂CO₃ and NaOH are injected continuously into contact with the continuous subterranean fluid stream that is rising up after being produced from the formation.

Referring generally to FIG. 1, a subterranean well 1 is shown having a production tube 2 with an annular space 3 defined between the walls of the subterranean well 1 and the production tube 2. The production fluid 4 while being continuously pumped up through the production tube 2 is scrubbed with an aqueous solution 6 that is continuously sprayed at a specified depth 7 from coil tubing 5 which is inside the production tube 2. The aqueous solution 6 mixes with the production fluid 4 creating a reaction product 8.

One advantage of the invention is the use of stoichiometry and pressure differences to ensure that after injection, the aqueous solution 6 does not reach the formation 9 of the well. If the solution 6 comes in contact with the formation 9, it could lead to damage, a change in the porous media properties, and reduction of hydrocarbon production rates from the well.

Permeation of either the NaOH or Na₂CO₃ into the formation is a key source of concern, and is carefully prevented as disclosed herein.

In a preferred embodiment of the invention, as a result of contact of Na₂CO₃ and H₂S, sodium sulfide is produced along with water according to the following reactions:

H₂S+Na₂CO₃→Na₂S+H₂0+CO₂

Na₂CO₃+2CO+H₂0→2HCOONa+CO₂

After the H₂S is consumed by Na₂CO₃ and produces sodium salts, the equilibrium is moved to the right in the reaction formula, resulting in the following reactions:

CO₂+H₂O→CO₃ ⁻+2H⁺

Na₂CO₃+2H₂O→Na⁺+H₂CO₃+OH⁻

OH⁻+2H⁺→H₂O

Thus, contact of H₂S and CO₂ with Na₂CO₃ ultimately results in water soluble sodium salts containing the sulfur and carbon from the acid gas, and these water soluble salts can be removed from the well in the water stream produced therefrom.

As mentioned above, it is a key concern to avoid contact between the solution and the formation, as the sodium-containing additives are known to cause formation damage. The problem of contacting the solution with the formation is therefore addressed, as mentioned above, through manipulation of process stoichiometry and through manipulation of differential pressure between the well and the formation.

With an understanding of the amount of acid gas present in the produced fluid, an amount of additive can be utilized such that the additive is entirely or at least substantially consumed chemically during the reaction, and therefore there is no additive remaining to contact the formation. This type of manipulation is referred to as controlling the additive through stoichiometry.

Alternatively, or in addition, the pressure within the well can be kept slightly lower than the pressure in the formation such that fluids are not forced into the formation, and thereby the solution does not actually contact the formation.

The aqueous solution (Na₂CO₃) is preferably injected at a concentration of active ingredient of less than about 11% weight for a typical formation fluid. The solution, which could be for example a 10% wt. solution, can be injected at rates, based on the incoming fluid flow, as follows. Approximately 12-30 cm³ (cc) of solution can be used per 1 ft³ of subterranean fluid containing between 1,000 and 4,000 ppm (0.1-0.4%) H₂S and/or between 10,000 and 30,000 ppm (1 and 3%) CO₂.

Further, approximately 25-30 cm³ (cc) of solution can be used per 1 ft³ of subterranean fluid containing between 10,000 and 20,000 ppm H₂S and/or between 40,000 and 50,000 ppm (4 and 5%) CO₂.

The above two paragraphs provide ranges for particular cases of how much aqueous solution (Na₂CO₃) is to be used in order to stoichiometrically avoid contact of the active ingredient (Na₂Co₃) of the solution with the formation.

Similar reactions occur when the active ingredient is NaOH. The important parameter, which can be determined by a person skilled in the art using known chemical relationships, is to introduce only as much sodium as will be consumed by reaction with the H₂S, such that the active ingredient is stoichiometrically presented from contacting the formation. This determination can preferably also involve a prior determination of the amount of H₂S in the produced reservoir fluids.

Thus, according to a preferred embodiment of the invention, a sample of formation fluid is obtained and analyzed to determine its H₂S content, preferably both H₂S and Co₂. This content can then be used, for example on a per volume basis, to determine a proper amount of active ingredient, or sodium, to introduce in the aqueous solution such that the active ingredient or sulfur will be consumed in reaction with the H₂S and CO₂ and thereby be stoichiometrically prevented from reaching the formation. In this way, the harmful components of H₂S and Co₂ are converted to water soluble products that are easily separated from water soluble products that are easily separated from water produced from the well, without exposing the well head, pipelines, etc., to corrosive H₂S and also while protecting the formation from being damaged by the active ingredient.

In accordance with a further embodiment of the invention, contact of the active ingredient in solution with the formation can be prevented by maintaining hydrostatic or dynamic pressure in the well at a lower level than formation pressure. This pressure imbalance, which should be small, keep fluid flow in the direction away from the formation and thereby keeps the solution and active ingredient from damaging the formation.

Excellent results have been obtained in accordance with the present invention, without exposing the formation to the expected significant damage due to contact with sodium-based scrubbing materials. Thus, in accordance with the present invention, a solution is provided whereby hydrocarbon fluids containing acid gases can be treated downhole to remove the acid gas before leaving the well, such that the well, surface facilities and pipelines are not exposed to acid gas.

It is to be understood that the invention is not limited to the illustrations described and shown herein, which are deemed to be merely illustrative of the best modes of carrying out the invention, and which are susceptible of modification of form, size, arrangement of parts and details of operation. The invention rather is intended to encompass all such modifications which are within its spirit and scope as defined by the claims. 

What is claimed is:
 1. A process for the removal of acid gas from fluid produced from a subterranean formation into a subterranean well comprising the steps of: producing fluid from the subterranean formation into an area selected from the group consisting of the subterranean well, a production tube in the well, an annular space defined therebetween, and combinations thereof so as to produce a production fluid containing acid gas; and introducing an aqueous solution of an active ingredient selected from the group consisting of Na₂CO₃, NaOH, and combinations thereof into said area, so as to contact the aqueous solution with the fluid as the fluid travels from a down-hole production tube to a surface well facility so as to contact said material with said acid gas and thereby remove said acid gas from the production fluid.
 2. The process as claimed in claim 1, wherein the aqueous solution is introduced via coil tubing to a specified depth in said area.
 3. The process as claimed in claim 1, wherein the aqueous solution is sprayed from a location along the production tube in order to cover a width of said area.
 4. The process as claimed in claim 3, wherein the solution is sprayed from multiple locations along the production tube in order to cover the width of said area.
 5. The process as claimed in claim 1, wherein the introducing step reacts the solution with said acid gas to produce sodium salts in a water solution and a hydrocarbon product which is substantially free of acid gases.
 6. The process as claimed in claim 1, wherein said aqueous solution does not reach said subterranean formation of said well.
 7. The process as claimed in claim 6, wherein said aqueous solution is kept from said subterranean formation by maintaining pressure in said area lower than formation pressure.
 8. The process as claimed in claim 1, wherein the solution is introduced in an amount such that the active ingredient is stoichiometrically prevented from reaching the formation.
 9. The process as claimed in claim 8, further comprising the steps of determining an amount of acid gas in the fluid, and selecting an amount of the active ingredient such that the active ingredient is less than what would be required to fully react with the amount of acid gas, whereby the active ingredient is stoichiometrically prevented from reaching the formation.
 10. The process as claimed in claim 1, wherein the solution is introduced at a ratio of 1 mole of active ingredient per mole of acid gas.
 11. The process as claimed in claim 1, wherein products of the introducing step are water soluble in a water production stream.
 12. The process as claimed in claim 1, wherein the producing step and the introducing step are continuous processes. 